A team of researchers recently published a paper in the journal Sensors and Actuators A: Physical that demonstrated the effectiveness of metal oxide thin film-coated evanescent wave-based fiber optic volatile organic compound (VOC) sensor devices in monitoring the VOC concentration for prognosis of disease severity.
Study: Metal oxide thin films coated evanescent wave based fiber optic VOC sensor. Image Credit: Zdenek Sasek/Shutterstock.com
Applications of Volatile Organic Compound Devices
Exhaled breath analysis has recently gained wide attention among the scientific community owing to its potential in precision medicine and early detection of diseases. This non-invasive analysis provides several advantages such as real-time quantitative/qualitative disease diagnosis and cost-efficiency.
Analysis of VOC concentration in exhaled breath can help in metabolic status monitoring and disease detection. However, detecting a specific VOC with a concentration in the range of ppb/ppm is difficult using conventional techniques.
In optical fiber sensors, the clad modified evanescent wave technique is typically applied to improve the sensing performance. In this technique, a small section of cladding is removed and replaced with a nanoparticle (NP) sensing layer. Although NP sensing layers are highly sensitive and easy to fabricate, they have less stability and low repeatability.
Metal oxide semiconductors (MOSs) are more suitable than NPs due to their high thermal and chemical stability and large surface-to-volume ratio. Specifically, n-type wide bandgap semiconductors such as tin oxide (SnO2) and zinc oxide (ZnO) were widely used as sensing layers for VOC detection as they have proper optical conditions, such as a higher refractive index, for evanescent sensing.
The sensing performance of metal oxides can be improved further by doping them with metallic NPs such as aluminum (Al).
In this study, researchers sputter-coated thin films of SnO2, ZnO, and aluminum-doped zinc oxide (AZO) over an unclad optical fiber core to fabricate VOC sensor probes, and investigated the performance of the sensor probes against several concentrations of VOCs. Additionally, wavelength and intensity shift interrogation techniques were performed to investigate the sensor probes against the target VOCs.
The Study
A 600 µm plastic-clad silica (PCS) core with 0.39 numerical aperture was utilized to fabricate the sensor probes. Initially, a 2 cm plastic-clad portion in the optical fiber was removed by immersing it for 60 min in an acetone solution, and then thin films of AZO, ZnO, and SnO2 were individually deposited on the unclad portion using radiofrequency (RF) magnetron sputtering.
Ellipsometer device, ultraviolet-visible (UV-Vis) spectroscopy, X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and scanning electron microscope (SEM) were used to characterize the deposited metal oxide thin films.
Spectral characteristics, intensity variation, and sensor response of the fabricated sensor probes were evaluated against different VOCs that include isopropanol (IPA), ethanol, acetophenone, and acetone at 0-250 ppm concentrations.
Observations
The SEM images demonstrated uniform sputtering deposition of the metal oxides on the unclad portion of the optical fiber. The surface grains of the sensing layers were also distributed uniformly, with the grain sizes of ZnO, AZO, and SnO2 films were measured as 55 nm, 70 nm, and 44 nm.
The thickness of the deposited SnO2, ZnO, and AZO thin films layer was 300 nm ± 3 nm. A wurtzite hexagonal structure was observed in the ZnO thin films, while a polycrystalline structure with a tetragonal phase was observed in the SnO2 films.
In the UV region at 265 nm wavelength, all thin films displayed a sharp absorption edge.
The optical band gaps of SnO2, AZO, and ZnO were 3.89 eV, 3.35 eV, and 3.48 eV, respectively. The output resistances of all sensor probes reduced after they were passed through a sensing chamber containing 250 ppm IPA.
All sensor probes functioned in the partial internal reflection condition. SnO2 thin films demonstrated higher imaginary and real parts of refractive index values compared to other thin films.
The refractive index of the ZnO, AZO, and SnO2 sensing layers was 2.1, 1.9, and 2.4 when they were exposed to different concentrations of VOCs compared to the plastic core refractive index of 1.453.
ZnO exhibited spectral peaks at 613 nm and 742 nm; AZO at 525 nm, 635 nm, and 735 nm; and SnO2 at 624 nm and 721 nm. The maximum change in peak 1 intensity was observed in the SnO2-coated sensor probe when all films were was exposed to VOCs. Additionally, both peaks of SnO2 displayed greater intensity variation to IPA than other VOCs. SnO2 showed the maximum wavelength shift of 2.4% to IPA among all films.
SnO2-coated sensor probe demonstrated a better sensor response compared to other thin films. Sensor response up to 21.2 % was achieved for IPA, with a recovery time and ideal response time of 21 s and 17 s, respectively. Grain size and porosity were acted as the key factors for higher sensor response.
SnO2 performance remained relatively stable for seven days, with an acceptable 6.6% error deviation from the first day. The detection limit of the SnO2-coated sensor probe was 300 ppb.
Taken together, the findings of this study demonstrated that metal oxide thin film-coated fiber optic VOC sensors, specifically SnO2-coated sensors, can effectively measure VOC concentrations in a wide range of biomedical applications.
Reference
A, Prasanth., S.R. Meher., Z.C. Alex. (2022) Metal Oxide Thin films coated Evanescent Wave based Fiber Optic VOC sensor. Sensors and Actuators: A. Physical https://www.sciencedirect.com/science/article/pii/S0924424722000978.
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